1. Field of Invention
The present invention relates to non-aqueous electrolytic solutions and electrochemical cells that contain such solutions. More particularly, this invention pertains to non-aqueous electrolytic solutions having a very low content of water and acids, and the preparation of such solutions. The invention further relates to a method of reducing at least one of the acid content and the water content in a non-aqueous electrolytic solution for use in an electrochemical cell.
2. Description of Related Art
Electric current producing cells such as batteries consist of pairs of electrodes of opposite polarity separated by electrolytic solution, which includes a solvent and a solute. The charge flow between electrodes is maintained by an ionically conducting solute, i.e., a salt. The non-aqueous electrolytic solutions, which are used in lithium and lithium ion batteries, are made by dissolving lithium salts in a variety of organic solvents. In particular, nonaqueous electrolytes comprising lithium hexafluorophosphate (LiPF6) exhibit very good electrochemical stability and conductivity. However, LiPF6 is not thermally stable and readily decomposes by hydrolysis, as set forth in the following well-known reactions:LiPF6→LiF+PF5  (1)LiPF6+H2O→2HF+LiF+POF3  (2)
Thermal decomposition of LiPF6 occurs at elevated temperatures (Reaction 1), and is accelerated in solution due to the reactions of PF5 and solvents. Hydrolysis (Reaction 2) generally occurs due to moisture and acidic impurities in the lithium salt and electrolytic solution. Accordingly, both water and hydrogen fluoride (HF) are undesirable in lithium and lithium-ion battery systems. The strong acid HF is especially harmful to batteries because it reacts with electrode active materials and corrodes the solid electrolyte interface (SEI), which results in poor battery performance. Thus the performance of such an electrolytic solution, and hence of a battery made therewith, is not optimal.
One method of removing acidic impurities in the electrolytes is to treat the electrolytes with a strong base and then maintain the electrolytes under basic conditions. U.S. Pat. No. 5,378,445 to Salmon et al. (hereby incorporated by reference into the present disclosure) describes the use of ammonia to prevent the acid-catalyzed decomposition of LiPF6. However, the presence of both ammonia and reaction products of ammonia, e.g. NH4F formed by the reaction of hydrogen fluoride and ammonia in the electrolytes may be detrimental to battery performance.